Impregnating saturating grade Kraft paper, or alpha-cellulose paper, with phenolic, melamine, epoxy, or polyester resin, for use in making decorative and industrial laminates is well known in the art, and taught, for example, by Alvino et al., in U.S. Pat. No. 4,327,143. Providing a quick, complete, and uniform impregnation of saturating grade Kraft paper, especially if it is a thick, high basis weight type, is a well recognized problem. Incomplete impregnation of the paper in a high speed process results from the high molecular weight of the impregnating resin, and the difficulty of having the resin flow into the pores of the fibrous sheet in a short time period. As the basis weight and caliper of the sheet increases, the difficulties of obtaining uniform, quick impregnation increase.
Both U.S. Pat. No. 4,044,185 and U.S. Pat. No. 3,648,358 describe high pressure decorative laminates. The body or core of the laminate is made of a plurality of phenol-formaldehyde impregnated Kraft paper sheets. It should be apparent that an increase in the thickness of the individual Kraft paper sheets of the core that could be thoroughly impregnated with phenolic resin, could reduce the number of sheets needed in the core. This improved productivity would, of course, require that thorough resin impregnation be obtained at the typical, high constant speed of production resin treaters, above about 500 ft./min.
In the standard method of impregnating laminating paper, described by Alvino et al., referred to above, the fibrous sheet is passed over an initial resin coated roller, to force resin into the sheet pore volume, and then through a resin bath operating at about 25.degree. C. to 30.degree. C. by means of immersed rollers. The travel path through the resin bath is usually from about 8 feet to 10 feet (2.4 to 3 meters), and the dwell time of a differential length of sheet is usually under 0.5 second in commercial operations, since travel rates are usually nominally constant at about 550 feet/minute (167.6 meters/minute). The excess resin is then removed by passing the wet sheet through a set of opposed nip rollers, after which the wet sheet is passed through a long drying oven to "B"-stage the resin. The "B"-staged sheet is then usually cut to appropriate size and can be used in the core of a high pressure laminate.
In order to produce a complete impregnation of thicker, higher basis weight paper, it would be necessary to increase the length of time in the resin bath, as by slowing the sheet travel rate or lengthening the bath, utilizing an immersed heater to increase the temperature of the resin substantially to reduce resin viscosity, or reducing the molecular weight of the resin. However, these solutions provide additional problems. Increasing retention time in the resin bath results in slower line speed, reduced productivity, and increased resin usage. Increasing the temperature is difficult due to buildup of a thermally insulating barrier of cured resin at the surface of the heating element and the eventual loss of heating efficiency. Reducing the molecular weight of the resin results in reduction in product properties and increased loss of resin solids during the subsequent drying operation.
Naundorf et al., in German Democratic Republic Pat. No. 124308, issued Feb. 16, 1977, proposed contacting the impregnating resin bath with one or more ultrasonic generators, and/or attaching one or more ultrasonic generators to the outside steel body of the immersion tank and transmitting the acoustic energy through the steel body to the impregnating resin. The ultrasonic radiation generally disclosed in Naundorf et al., presumably provides improved resin penetration into the interstices of the fibrous sheet.
While Naundorf et al. and others have suggested the potential of improved resin penetration through the general use of ultrasonic energy, no one appears to have addressed the specific problem of providing thoroughly impregnated high basis weight paper in the treatment of such paper in high-speed treaters, particularly, sheets having a nominal width of about 50 inches travelling at speeds above about 500 ft./min. Kraft papers having a basis weight of up to about 150 lbs./3,000 sq. ft. can be properly treated most of the time in treaters that do not use ultrasonic energy, with problems occurring intermittently but mostly in January and February when colder temperatures raise resin viscosity. Such poor penetration is characterized by varnish coating the surface of the Kraft paper but not thoroughly impregnating it. Laminates made from such poorly impregnated paper have poor blister resistance and are not commercially acceptable.
It should be understood that unless the ultrasonic energy can be designed to solve this specific problem, its use would be counter-productive. The cost of equipment and energy would be wasted if only an immeasurable or minor improvement is obtained. It should also be understood that small scale tests in beakers and laboratory sized equipment cannot be easily translated to effective production solutions at the scale and speeds described.